. >>> president obama has talked about red lines. >> yes. >> do you think bashar al assad has crossed a red line? >> not just for one time. many, many times, he crossed the red line. >> that was the military leader of rebels in syria talking exclusively to our richard engel about allegations that forces loyal to president assad has used chemical weapons in their desperate battle to king to power. we're joined by richard and steve. rich, the president said that he hasn't seen a clear chain of custody that tells us who's been using chemical weapons in syria. but it seems like the chain of custody is all too clear for the rebel leader that you interviewed. >> he said the chain of custody is clear because not all the evidence they have uncovered, but because no one other than bashar al assad's regime could have used chemical weapons. he said the weapons were three times dropped by fighter jets, the rebels don't have fighter jets and that one time were delivered by a scud and they don't have those either. based on the description of the evidence, these are hair samples, blood samples, urin